A miter joint is a joint made by beveling each of two surfaces to be joined, for example at a 45° angle, to form a corner the included angle of which is the sum of the two bevel angles. Two commonplace examples of miter joints are found in the trim molding placed at the upper ends of walls in many rooms, and at the corners of picture frames. Miter joints provide a finished and esthetically-pleasing appearance and may improve the structural strength or rigidity of the joined pieces in some applications.
In order to produce an effective miter joint, it is necessary to cut the pieces being joined along a proper angle. For example, if the pieces join each other to form a right angle, each piece must be mitered along a 45° angle; any significant deviation from the desired angle when cutting either piece will result at best in a miter joint having a sloppy or unsightly appearance, and at worst in a defective piece that cannot be used for the intended purpose.
Carpenters typically use miter boxes for guiding a saw at the proper angle while making beveled cuts in boards. A typical miter box may be described as a U-shaped box of wood or plastic, open at the ends and having pre-cut guide grooves in the sides to guide a saw blade at selected angles of cut. A sacrificial baseboard is placed in the bottom of the miter box to protect the bottom and the teeth of the saw blade, and then a workpiece is laid in the box and held tightly against one side of the box. A beveled cut is then made using the guide slots at the desired angle.
Although miter boxes are commonplace for use by carpenters, problems are associated with their use. For one thing, commercially-available miter boxes usually are limited to cuts of 90°, 60°, 45° and 30°. Furthermore, miter boxes are relatively bulky and seldom at hand for the user. Such boxes typically have a dusty, cut-up bottom board in them, and the guide slots in the sides of the miter box are quickly widened by the back-and-forth cutting movement of handsaws through the guide slots. This widening of the guide slots means that the miter box becomes less precise in guiding a miter joint of the desired angle, leading to the eventual need to discard and replace a worn-out miter box.
Radial-arm saws allow infinite cut-angle settings for producing miter joints. However, such saws are expensive and require electricity to operate, and thus are not practical for the casual user or for someone preferring a portable miter tool.